This invention relates to polyetherimide blends, and more particularly to the preparation of compatible reinforced blends having low viscosity and high workability.
Thermoplastic polyetherimides are known polymers having many beneficial properties, especially high resistance to thermal distortion. However, their ductility and flow properties are often poor. In particular, they frequently have such high viscosities when worked (as by extrusion or molding) that they are very difficult to form. In addition, fillers, fibers and the like can be added to polyetherimides, but such addition adversely affects viscosity and make the compositions more difficult to process.
Thermotropic liquid crystalline polymers, particularly polyesters, have been considered as viscosity modifiers for thermoplastic resins. They have a tendency to form highly oriented structures when subjected to deformations in the melt state, and their rod-like molecular conformation and chain stiffness tend to make their properties similar in some respects to those of solid fibers with respect to reinforcement capability. In addition, liquid crystalline polyesters have low viscosities, low thermal expansion coefficients, good barrier properties to solvents and high thermal stability. All of these properties make such polyesters excellent candidates for use as viscosity modifiers for polyetherimides, capable of maintaining relatively low viscosity when compared to that of the neat polyetherimide.
A problem in polyetherimide-liquid crystalline polyester blends, however, is the incompatibility of the two polymers, resulting in poor adhesion between the polyetherimide and the liquid crystalline polyester. This incompatibility may lead to problems such as phase separation as demonstrated by delamination or the formation of skin-core layered structures during molding operations, particularly injection molding. By "delamination" is meant visually observed formation of poorly adhered layers at the surface or in the bulk of a test sample. Incompatibility may also lead to poor mechanical properties and marginal surface appearance (streaking, pearlescence, etc.) under abusive molding conditions; that is, conditions including high molding and/or extrusion temperatures or long residence times in the extruder and/or in the molding machine.
Japanese Kokai 1/315465 and 2/041355 disclose blends comprising polyetherimides, non-liquid crystalline polyesters such as poly(ethylene terephthalate) and ethylene-glycidyl methacrylate copolymers. Said copolymers improve the physical properties of these blends, particularly impact strengths as measured by the Izod and Dynatup methods. Blends of polyetherimides with non-liquid crystalline polyesters are not, however, characterized by incompatibility as are those with liquid crystalline polyesters, and thus there is no need for the presence of a polyepoxy compound as a compatibilizing agent.
U.S. Pat. No. 5,324,795 discloses blends of liquid crystalline polymers including polyesters, non-liquid crystalline polymers and "thickeners" which may be compounds containing epoxy groups. There is no specific disclosure of polyetherimides and no indication that the "thickener" serves any purpose other than increasing the melt viscosity, an effect which would be undesirable in the present context.
It would be advantageous, therefore, to develop compatibilization strategies for blends comprising a major proportion of thermoplastic polyetherimide and a minor viscosity modifying proportion of liquid crystalline polyester.